1. Field of the Invention
This invention relates to a hollow cast article having a slit and a method and apparatus for the production thereof. More particularly, this invention relates to a casting technique usable in the production of a hollow cast article having a slit, particularly optical connector parts such as a sleeve for optical connector ferrules and a conversion sleeve for ferrules having different diameters, by casting, particularly by metal mold casting of an amorphous alloy (metal glass).
2. Description of the Prior Art
As a typical example of a hollow cast article which has a slit and which requires high dimensional accuracy, optical connector parts such as a sleeve for optical connector ferrules and a conversion sleeve for ferrules having different diameters may be cited.
A sleeve for optical connector ferrules is used to connect ferrules each having an optical fiber already inserted and joined therein by abutting ends of the ferrules against each other. In general, the connection of the optical fibers are attained by inserting the ferrules into a split sleeve through the opposite ends thereof and then abutting the ends of the ferrules against each other, thereby allowing the leading ends of the optical fibers to be abutted and connected in the axial alignment state (see Japanese Patent Application, KOKAI (Early Publication) No. (hereinafter referred to briefly as “JP-,A”) JP-6-27348,A and JP-10-311923,A, for example). On the other hand, a conversion sleeve for ferrules having different diameters is used to connect ferrules having different diameters by abutting ends of the ferrules against each other (see JP-9-90169,A, for example).
In the production of the hollow cast article having a slit, such as the sleeve for optical connector ferrules and the conversion sleeve for ferrules having different diameters as mentioned above, if the draft angle of a core which forms a hollow portion is in the range of 0° to 5°, the problem will be incurred that the core cannot be drawn out from a cast product during the ejection step. If the core is forcedly drawn out from the cast product, another problem will be incurred that part of a cast material adheres to the surface of the core and the resultant cast product has the coarsened inside surface. Particularly when the casting material is an amorphous alloy (metal glass), it exhibits excellent transferability capable of faithfully reproducing the contour of a cavity of a mold. As a result, it is possible to produce an amorphous alloy cast product satisfying dimensional prescription, dimensional accuracy, and surface quality. However, this means that the core is in the state of shrink-fit into the cast product due to the thermal shrinkage thereof and there is little gap between the surface of the cavity of the mold and the cast material. This fact, therefore, poses the problem that when the core is drawn out from the cast product, the core rubs against the inside surface of the hollow cast product and part of the cast material adheres to the entire surface of the core in the state of thin film due to the properties of the amorphous alloy that it is hard but easily deformable (remarkable plastic deformation under multi-axial stress). As a result, scratch marks remain in the inside surface of the hollow cast product.
In such a case, the hollow cast article has a coarsened inside surface (increased surface roughness) even when the surface of the core to be used is smooth. This is a fatal defect to the hollow cast article such as optical connector parts mentioned above which require the smoothness of the inside surface.